1. Field of the Invention
The present invention is directed to communication systems and networks and is particularly directed to such systems and networks which use multi-channel, multi-frequency protocols such as orthogonal frequency division multiplexing and discrete multi-tone protocols.
2. Background of the Related Art
Orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) are two closely related formats which have become popular as communication protocols. Systems of this type take a relatively wide bandwidth communication channel and break it into many smaller frequency sub-channels. The narrower sub-channels are then used simultaneously to transmit data at a high rate. These techniques have advantages when the communication channel has multi-path or narrow band interference.
The following discussion of the prior art and the invention will address OFDM systems; however, it will be understood that the invention is equally applicable to DMT systems (as well as other types of communication systems) with only minor modifications that will be readily apparent to those skilled in the art.
A functional block diagram of a typical OFDM transmitter is shown in FIG. 1. Here, an incoming stream 10 of N symbols d0, d1 . . . dN−1 is mapped by a serial-to-parallel converter 20 over N parallel lines 30, each line corresponding to a particular subcarrier within the overall OFDM channel. An Inverse Fast Fourier Transform circuit 40 accepts these as frequency domain components and generates a set 50 of time domain subcarriers corresponding thereto. These are converted by a parallel-to-serial converter 60. Due to the characteristics of the inverse Fourier transform, although the frequency spectra of the subcarrier channels overlap, each subcarrier is orthogonal to the others. Thus, the frequency at which each subcarrier in the received signal is evaluated is one at which all other signals are zero.
A functional block diagram of the corresponding OFDM receiver is shown in FIG. 2. Here, an OFDM signal is received and converted into multiple time domain signals 210 by a serial-to-parallel converter 220. These signals are processed by a Fast Fourier Transform processor 230 before being multiplexed by parallel-to-serial converter 240 to recover the original data stream 250.
Systems such as OFDM and DMT systems either do not share the main channel with other users at all (e.g., when they are implemented using a telephone modem), or share the channel in time (e.g., when implemented in TDMA and CSMA schemes); thus, their flexibility and ease of use is limited. Sharing the channel in time (i.e., allowing only one user to transmit at a time) has two serious disadvantages. First, to maintain high throughput, all nodes sharing the channel must operate at a high data rate, and therefore be equally complex; thus, no less-complicated processing circuitry which might otherwise be used with low data rate channels can be employed. Second, a user who actually desires a low data rate must send data as very short high speed bursts over the network. In order to overcome propagation loss in the path, such a node must transmit at a high peak power because the transmit power is proportional to the peak data rate. Again, economies inherent in the low data rate processing cannot be exploited.
As a practical example, the IEEE 802.11 a communication standard specifies transmission with 52 sub-channel frequencies. This requires substantial signal processing; a high transmit power while active to achieve significant range; a large peak-to-average ratio while actively transmitting; high resolution ADCs and DACs; and very linear transmit and receive chains. While such complicated hardware allows transmission up to 50 Mb/s, this level of performance is overkill for something like a cordless phone, which only requires roughly a 32 kb/s transmission rate.
In connection with the peak-to-average ratio, note that for 52 sub-channels, while transmitting the peak-to-average ratio of the signal is 522/52=52 in power. Therefore, to avoid distortion of the signal, the power amplifier must be substantial enough to provide far more instantaneous power than is required on average. Since the peak-to-average ratio is directly proportional to the number of sub-channels, building a lower capacity unit that uses fewer carriers can substantially decrease the costs of such a device.